The spot uses a satellite transmitter to transmit it's GPS location to a satellite. The information is then transferred to a ground station where it can be looked up on the web.

Cell phones don't have satellite transmitters. They can only connect to cell towers and nearby wifi terminals. If you are in the mountains you are often too far away from cell towers or wifi terminals the cell phone cannot send the information out. you won't find a app that will allow a cell phone to connect to a satellite.

As mentioned, using your data connection while hiking is probably not a reliable/battery efficient way to do things, unless you're sure there will be tower coverage. Google latitude might be a decent way to share your location while in data range.

If you have data coverage for your phone (which doesn't happen where I usually hike) you can use Google Lattitude and it will update your position regularly. I wouldn't count on this though…It's good if you're in a city park but data service typically doesn't reach most remote sites effectively.